The search for the origins of the Covid pandemic that has killed millions and crippled economies is at a standstill even as time is running out, scientists charged with the task by the UN warned Wednesday.

An initial report by the World Health Organization (WHO) team based on a January mission to Wuhan, China -- ground zero of the global pandemic -- concluded that the SARS-CoV-2 virus probably jumped from bats to humans via an intermediate animal.

A competing hypothesis that the virus leaked from a specialised virology lab in Wuhan was deemed "extremely unlikely".

But in a comment in the journal Nature, the scientists said that mission was only intended as a "first step in a process that has stalled."

"The search for the origins of SARS-CoV-2 is at a critical juncture," they wrote.

"The window of opportunity for conducting this crucial enquiry is closing fast."

Tracing the biological trail back to the earliest pockets of the disease becomes more difficult as evidence disappears or becomes corrupted.

The statement comes less than two weeks after the WHO, in a bid to revive the probe, urged China to hand over information on the earliest COVID-19 cases.

This should include Covid data for 174 infections identified in December 2019 that China failed to share during the initial investigation, the WHO experts said.

WHO investigators said "it was agreed" at the time that a second phase of research would fill in this gap.

But China pushed back against the WHO request earlier this month, saying the January investigation should suffice and that calls for further data were motivated by politics, not science.

'Diminishing Returns'

Beijing has especially bridled at the suggestion that the virus might have escaped from the Wuhan virology lab.

On Tuesday, US intelligence agencies presented President Joe Biden with a report looking at both the animal transmission and "lab-leak" hypotheses. The findings were described as inconclusive.

In its comment, the WHO team notes that current data does not support the lab-leak scenario.

None of six priorities for further research alluded to this possibility.

Rather, the scientists emphasised the need to trace the earliest cases of Covid through disease reporting and antibody surveys, inside and outside China.

They also called for further investigation of wildlife farms and wild bats.

"As SARS-CoV-2 antibodies wane, so collecting further samples and testing people who might have been exposed before December 2019 will yield diminishing returns," they said.

It added that many of the wildlife farms of interest for study have been closed and their livestock killed.